The present invention is directed to polyaxial bone screws for use in bone surgery, particularly spinal surgery, and cooperating elongate connecting members that are at least somewhat plastically deformable. Such screws have a receiver or head that can swivel about a shank of the bone screw, allowing the receiver to be positioned in any of a number of angular configurations relative to the shank.
Many spinal surgery procedures require securing various implants to bone and especially to vertebrae along the spine. For example, elongate members, such as solid rigid rods are often utilized that extend along the spine to provide support to vertebrae that have been damaged or weakened due to injury or disease. Such elongate members must be supported by certain vertebrae and support other vertebrae.
The most common mechanism for providing vertebral support is to implant bone screws into certain bones which then in turn support the elongate member or are supported by the elongate member. Bone screws of this type may have a fixed head or receiver relative to a shank thereof. In the fixed bone screws, the head cannot be moved relative to the shank and the rod or other elongate member must be favorably positioned in order for it to be placed within the head. This is sometimes very difficult or impossible to do. Therefore, polyaxial bone screws are commonly preferred.
Polyaxial bone screws allow rotation of the receiver about the shank until a desired rotational position of the receiver is achieved relative to the shank. Thereafter, a rod or other elongate connecting member can be inserted into the receiver and eventually the rod and the receiver are locked or fixed in a particular position relative to the shank.
A variety of polyaxial or swivel-head bone screw assemblies are available. One type of bone screw assembly includes an open head or receiver that allows for placement of a rod or other elongate member within the receiver. A closure top or plug is then used to capture the rod in the receiver of the screw. Thus, the closure top or plug pressing against the rod not only locks the rod in place but also locks the bone screw shank in a desired angular position with respect to the receiver. A draw back to such a system occurs when the rod or other elongate connecting member is made from a material that exhibits creep or viscoelastic behavior. Creep is a term used to describe the tendency of a material to move, flow or to deform permanently to relieve stresses. Material deformation occurs as a result of long term exposure to levels of stress that are below the yield or ultimate strength of the material. Rods and other longitudinal connecting members made from polymers, such as polyetheretherketone (PEEK), especially pure PEEK and rubbers, have a greater tendency to exhibit creep, than, for example metals or metal alloys, such as stainless steel, titanium and nickel titanium (commonly referred to by its trade name Nitinol). When a rod or other longitudinal connecting member exhibits creep deformation over time, the closure top may no longer tightly engage the connecting member. This in itself is not necessarily problematic. However, such loosening also results in loosening of the frictional engagement between the receiver and the bone screw shank that locks the angular orientation of the shank with respect to the receiver. Body movement and stresses may then result in undesirable pivoting of the shank with respect to the receiver causing mis-alignment, greater stress and further loosening of the various polyaxial bone screw components.
It is known to equip a bone screw assembly with an upper and/or a lower pressure insert located within the receiver for engaging and closely holding one or more surfaces of a rod or other longitudinal connecting member. Nested closure tops are known in the art that include a fastening portion having an outer thread for engaging an interior threaded surface of the receiver arms and also an inner thread for engaging a threaded set screw. The fastening portion of the closure top is disposed within the receiver arms but does not operationally abut against a rod or other longitudinal connecting member. Rather, the fastening portion abuts against one or more pressure inserts also located in the receiver that in turn engage a polyaxial bone screw mechanism so that the shank may be set and locked at a desired angular position with respect to the receiver prior to locking the rod or other longitudinal connecting member in place by direct or indirect pressure thereon by the independently rotatable inner set screw. However, one of the drawbacks to such a polyaxial implant is that to maintain a relatively non-bulky, low profile assembly, the numerous inserts and other component parts of such an assembly have thin walls that provide the desired low profile but otherwise lack sufficient strength to withstand the stresses placed thereon by the human body and other component parts of the bone screw assembly. In order to remedy this problem, it is further known in the art to equip such an assembly with an outer nut (located outside of the receiver arms) to keep the arms of the receiver from splaying and aid in securely holding any inserts and other small component parts within the receiver of the bone screw assembly so that the parts do not loosen or disassemble within the body. However, such a nut adds undesirable bulk to the assembly and may lead to undesirable interference with vertebrae and other medical implants.